github.com/afumu/libc@v0.0.6/musl/src/string/strstr.c (about)

     1  #include <string.h>
     2  #include <stdint.h>
     3  
     4  static char *twobyte_strstr(const unsigned char *h, const unsigned char *n)
     5  {
     6  	uint16_t nw = n[0]<<8 | n[1], hw = h[0]<<8 | h[1];
     7  	for (h++; *h && hw != nw; hw = hw<<8 | *++h);
     8  	return *h ? (char *)h-1 : 0;
     9  }
    10  
    11  static char *threebyte_strstr(const unsigned char *h, const unsigned char *n)
    12  {
    13  	uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8;
    14  	uint32_t hw = (uint32_t)h[0]<<24 | h[1]<<16 | h[2]<<8;
    15  	for (h+=2; *h && hw != nw; hw = (hw|*++h)<<8);
    16  	return *h ? (char *)h-2 : 0;
    17  }
    18  
    19  static char *fourbyte_strstr(const unsigned char *h, const unsigned char *n)
    20  {
    21  	uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8 | n[3];
    22  	uint32_t hw = (uint32_t)h[0]<<24 | h[1]<<16 | h[2]<<8 | h[3];
    23  	for (h+=3; *h && hw != nw; hw = hw<<8 | *++h);
    24  	return *h ? (char *)h-3 : 0;
    25  }
    26  
    27  #define MAX(a,b) ((a)>(b)?(a):(b))
    28  #define MIN(a,b) ((a)<(b)?(a):(b))
    29  
    30  #define BITOP(a,b,op) \
    31   ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
    32  
    33  static char *twoway_strstr(const unsigned char *h, const unsigned char *n)
    34  {
    35  	const unsigned char *z;
    36  	size_t l, ip, jp, k, p, ms, p0, mem, mem0;
    37  	size_t byteset[32 / sizeof(size_t)] = { 0 };
    38  	size_t shift[256];
    39  
    40  	/* Computing length of needle and fill shift table */
    41  	for (l=0; n[l] && h[l]; l++)
    42  		BITOP(byteset, n[l], |=), shift[n[l]] = l+1;
    43  	if (n[l]) return 0; /* hit the end of h */
    44  
    45  	/* Compute maximal suffix */
    46  	ip = -1; jp = 0; k = p = 1;
    47  	while (jp+k<l) {
    48  		if (n[ip+k] == n[jp+k]) {
    49  			if (k == p) {
    50  				jp += p;
    51  				k = 1;
    52  			} else k++;
    53  		} else if (n[ip+k] > n[jp+k]) {
    54  			jp += k;
    55  			k = 1;
    56  			p = jp - ip;
    57  		} else {
    58  			ip = jp++;
    59  			k = p = 1;
    60  		}
    61  	}
    62  	ms = ip;
    63  	p0 = p;
    64  
    65  	/* And with the opposite comparison */
    66  	ip = -1; jp = 0; k = p = 1;
    67  	while (jp+k<l) {
    68  		if (n[ip+k] == n[jp+k]) {
    69  			if (k == p) {
    70  				jp += p;
    71  				k = 1;
    72  			} else k++;
    73  		} else if (n[ip+k] < n[jp+k]) {
    74  			jp += k;
    75  			k = 1;
    76  			p = jp - ip;
    77  		} else {
    78  			ip = jp++;
    79  			k = p = 1;
    80  		}
    81  	}
    82  	if (ip+1 > ms+1) ms = ip;
    83  	else p = p0;
    84  
    85  	/* Periodic needle? */
    86  	if (memcmp(n, n+p, ms+1)) {
    87  		mem0 = 0;
    88  		p = MAX(ms, l-ms-1) + 1;
    89  	} else mem0 = l-p;
    90  	mem = 0;
    91  
    92  	/* Initialize incremental end-of-haystack pointer */
    93  	z = h;
    94  
    95  	/* Search loop */
    96  	for (;;) {
    97  		/* Update incremental end-of-haystack pointer */
    98  		if (z-h < l) {
    99  			/* Fast estimate for MIN(l,63) */
   100  			size_t grow = l | 63;
   101  			const unsigned char *z2 = memchr(z, 0, grow);
   102  			if (z2) {
   103  				z = z2;
   104  				if (z-h < l) return 0;
   105  			} else z += grow;
   106  		}
   107  
   108  		/* Check last byte first; advance by shift on mismatch */
   109  		if (BITOP(byteset, h[l-1], &)) {
   110  			k = l-shift[h[l-1]];
   111  			if (k) {
   112  				if (k < mem) k = mem;
   113  				h += k;
   114  				mem = 0;
   115  				continue;
   116  			}
   117  		} else {
   118  			h += l;
   119  			mem = 0;
   120  			continue;
   121  		}
   122  
   123  		/* Compare right half */
   124  		for (k=MAX(ms+1,mem); n[k] && n[k] == h[k]; k++);
   125  		if (n[k]) {
   126  			h += k-ms;
   127  			mem = 0;
   128  			continue;
   129  		}
   130  		/* Compare left half */
   131  		for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
   132  		if (k <= mem) return (char *)h;
   133  		h += p;
   134  		mem = mem0;
   135  	}
   136  }
   137  
   138  char *strstr(const char *h, const char *n)
   139  {
   140  	/* Return immediately on empty needle */
   141  	if (!n[0]) return (char *)h;
   142  
   143  	/* Use faster algorithms for short needles */
   144  	h = strchr(h, *n);
   145  	if (!h || !n[1]) return (char *)h;
   146  	if (!h[1]) return 0;
   147  	if (!n[2]) return twobyte_strstr((void *)h, (void *)n);
   148  	if (!h[2]) return 0;
   149  	if (!n[3]) return threebyte_strstr((void *)h, (void *)n);
   150  	if (!h[3]) return 0;
   151  	if (!n[4]) return fourbyte_strstr((void *)h, (void *)n);
   152  
   153  	return twoway_strstr((void *)h, (void *)n);
   154  }